1. Field of the Invention
The present invention relates to a method of adjusting a focus of an imaging optical system which formns an image of an interference image produced by an interference optical system and containing information about a shape of an object under inspection.
The invention also relates to a shape measuring device and an interference microscope for measuring, checking and monitoring a fine object by using the focus adjusting method.
2. Related Art Statement
Various types of shape measuring devices for measuring and checking objects have been proposed. For instance, one known type device has been disclosed in D. Malacara, "Optical Shop Testing", John Wiley and Sons, New York (1978). Particularly, a phase measurement using the fringe scan has been widely used in the measurement of a fine structure of an object, because depressions and protrusions of an object surface can be measured with a precision which is higher than a hundredth of a wavelength.
C. Bouwhius et al have reported in "Principles of Optical Disc Systems", Intern. Trens in Optic, Acad. Press (1991) that when an object surface has a step, diffracted light contains a phase jump (singular point) near the step. This singular point has a very small size, and is generated not only at the step, but also in a close vicinity of a point at which an optical property of an object is discontinuous. For instance, at a boundary of two different optical materials, a singular point occurs. Therefore, by measuring a position of a singular point with high precision, the optical discontinuous point can be measured precisely.
FIG. 1 is a schematic view showing a known shape measuring device. This shape measuring device utilizes the Twyman-Green interferometer. A parallel coherent light beam emitted by a laser 1 is expanded by a beam expander 2. The expanded parallel laser beam is made incident upon an interference optical system 3 formed by a half mirror and is divided thereby into an inspection laser beam which is directed toward an object 4 under inspection along an inspection optical path 5 and a reference laser beam which is directed toward a reference body 6 along a reference optical path 7. These laser beams are reflected by the object 4 and reference body 6, respectively and are made incident again upon the interference optical system 3 along the optical paths 5 and 7, respectively. At the interference optical system 3, these laser beams are composed to produce a composite laser beam due to the interference. The composite laser beam in then made incident upon an objective lens 8 and a composite image of the object 4 and reference body 6 in formed on an image sensing device 9. An image signal obtained by the image sensing device 9 is supplied to an image display device 11 via a controller 10 and an interference image is displayed thereon.
In the interference image displayed on the image display device 11, there is produced interference fringes in accordance with a local difference in an optical path length between the inspection optical path 5 and the reference optical path 7. Therefore, during the time that the reference body 6 is moved in a direction of an optical axis by driving a phase modulator 12 from the controller 10 to vary the difference in optical path length finely, a plural number of interference images are picked-up by the image sensing device 9. Then, a phase distribution in a vicinity of a surface of the object 4 can be calculated from the interference images. Methods of calculating the phase distribution from a plural number of interference images obtained by using the fringe scan have been described in detail in Catherine Creath, "PHASE-MEASUREMENT INTERFEROMETRY TECHNIQUES", Progress in Optics XXVI, Amsterdam 1988, pp. 350-393 and JP-A 5-232304.
However, in the known shape measuring devices, there is a problem in the detection of the singular point in a vicinity of the surface of the object under inspection. That is to say, the phase distribution is very sensitive to a focus of the objective lens with respect to the object under inspection. This has been explained in D. M. Gale et al, "Linnik Microscope Imaging", Applied optics, 35 (1996), pp. 131-148. For instance, in this article, the objective lens has a numerical aperture NA=0.9 and the light beam has a wavelength .lambda.=0.633 .mu.m. Then, a focal depth .DELTA. of the objective lens becomes .DELTA.=.lambda./2NA.sup.2 =0.4 .mu.m. However, even if a focus is moved only by a very small distance such as 0.1 .mu.m, the observed phase distribution might change greatly.
There have been also proposed various types of interference microscopes including interferometers and microscopes for measuring and checking fine objects. Particularly, there has been proposed an interference microscope for measuring a phase distribution in the vicinity of a surface of a fine object and inspecting the object by detecting a phase jump (singular point). Such an interference microscope has a higher resolution than a conventional optical microscope, and has been described in, for instance, V. P. Tychinsky, "ON SUPERRESOLUTION OF PHASE OBJECTS", Optics Communications, 74 (1989), pp. 41-45, and the above mentioned D. M. Gale et al, "Linnik Microscope Imaging", Applied Optics, 35 (1996), pp. 131-148.
FIG. 2 is a schematic view illustrating a known interference microscope of Michelson type. A mono-chromatic light beam emitted by a mono-chromatic light source 21 such as a laser is made incident upon an object 25 under inspection by means of condenser lens 22, half mirror 23 and objective lens 24. A part of the light beam is made incident upon a reference body 27 by means of an interference optical device 26 such as a half mirror provided between the objective lens 24 and the object 25. Light beams reflected by the object 25 and reference body 27 are composed by the interference optical device 26 and a composite image is formed as an enlarged image by means of the objective lens 24 and half mirror 23 on an image sensing device 28 such as a CCD. An output image signal from the image sensing device 28 is displayed on an image display device 30 by means of a controller 29.
On the image sensing device 28, interference fringes are formed in accordance with a local difference in optical path length between an inspection optical path 31 and a reference optical path 32. Therefore, as explained above with reference to FIG. 1, upon driving a phase modulator 33 to move the reference body 27 in a direction of optical axis, a plural number of interference images are taken by the image sensing device 28. Then, the phase distribution in the vicinity of the surface of the object 25 can be calculated from the interference images.
FIG. 3 shows a known interference microscope of Linnik type and FIG. 4 illustrates a known interference microscope of Mirau type. In FIGS. 3 and 4, the controller 29 and display device 30 are omitted for the sake of simplicity. In these interference microscopes, the light beam emitted by the laser 21 is focused on the pupil of the objective lens by means of two positive lenses 35a and 35b and a pin hole 36 provided therebetween, and thus the laser beam is made incident upon the object 25 as a plane wave.
The Linnik type interference microscope shown in FIG. 3 is generally used under such a condition that the Michelson type interferometer could not be provided owing to a small working distance. The half mirror constituting the interference optical device 26 is arranged above an inspection objective lens 24a, and a reference objective lens 24b is arranged in the reference optical path 32 at a position which is conjugate with a position of the inspection objective lens 24a in the inspection optical path 31. In FIG. 3, the half mirror of the interference optical device 26 also serves as the half mirror for introducing the laser beam into the objective lenses 24a and 24b, but a separate half mirror may be provided for this purpose.
In the Mirau type interference microscope depicted in FIG. 4, the laser beam emitted by the laser 21 is made incident upon the object 25 under inspection by means of positive lens 35a, pin hole 36, positive lens 35b, half mirror 37 and objective lens 24, the interference optical device 26 formed by the half mirror is arranged perpendicularly to the optical axis between the objective lens 24 and the object 25 under inspection, and the reference body 27 is arranged at a position which is conjugate with the object 25 with respect to the interference optical device 26. In this type of interference microscope, when the interference optical device 26 and reference body 27 are fixed with respect to the objective lens 24, the optical path length may be changed by varying a distance between the objective lens 24 and the object 25 under inspection for deriving the phase distribution from a plural number of interference images.
To the above mentioned interference microscopes shown in FIGS. 2-4, the above mentioned analysis for the electric field is equally applied. That is to say, the phase distribution changes abruptly in the vicinity of the singular point even if the focus of the objective lens moves over a distance which is shorter than the focal depth. Therefore, in order to measure a shape of the object on the basis of the singular point, the focus of the objective lens should be brought into the singular point, which does not always appear on the object surface.
In the known shape measuring devices as well as in the known interference microscopes, the position of the objective lens is manually adjusted such that the bright field image can be seen clearly and sharply or the position of the objective lens is automatically adjusted by means of an automatic focussing device. In either case, there always be introduced an error substantially equal to the focal depth of the objective lens. Due to this error, a measurement of the object under inspection could not always be performed at a position at which a singular point appears, and thus it is practically difficult to obtain reliable measurement results having high reproducibility and precision.